//不要给我WA WA 叫
//一定AC
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <set>
#include <map>
#include <cstring>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#include <iomanip>
#include <climits>
#define int long long 
#define x first 
#define y second 
using namespace std;
const int N = 1e5 + 10;
int a[N];
void solve()
{
    string s;
    cin >> s;
    int cnt[26] = {0};
    for(int i = 0; i < s.size(); i++)
    {
         cnt[s[i] - 'a']++;
    }
    for(int i = 0; i < 26; i++)
    {
        if(cnt[i] == 1)
        {
            cout << char('a' + i) << endl;
            return; 
        }
    }
    cout << "no" << endl;
}
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr),cout.tie(nullptr);
    int t = 1;
    while(t -- )  solve();
    return 0;
}
